1. Field of the Invention
The present invention relates to a semiconductor device having circuits structured by thin film transistors (hereafter referred to as “TFTs”), and to a method of manufacturing a semiconductor device. The present invention relates, for example, to electro-optical devices, typically liquid crystal display devices, and to electronic equipment in which the electro-optical devices are installed as parts. Note that the term semiconductor device as used throughout this specification indicates general devices capable of functioning by utilizing semiconductor characteristic, and that the aforementioned electro-optical devices and electronic equipment fall within the category of semiconductor devices.
2. Description of the Related Art
Techniques of crystallizing, and increasing crystallinity of, an amorphous semiconductor film formed on an insulating substrate such as glass by performing heat treatment, laser annealing, or both heat treatment and laser annealing have been widely researched in recent years. Silicon is often used in the semiconductor film.
Crystallized semiconductor films obtained in accordance with the above techniques are referred to as crystalline semiconductor films. The crystalline semiconductor films have extremely high mobility comparison with amorphous semiconductor films. A monolithic type liquid crystal electro-optical device (a semiconductor device in which thin film transistors (TFTs) for a pixel driver and a driver circuit are manufactured on one substrate) which cannot be achieved using semiconductor devices manufactured by conventional amorphous semiconductor films, for example, can therefore be manufactured if crystalline semiconductor films are utilized.
Crystalline semiconductor films are thus semiconductor films having extremely good characteristics compared to amorphous semiconductor films, and this is the reason the above stated research is being carried out. For example, it is necessary to have a heat treatment temperature equal to or greater than 600° C., and a heat treatment time equal to or greater than 10 hours, preferably equal to or greater than 20 hours, when performing crystallization of an amorphous semiconductor film by using heat treatment. Substrates which can withstand these crystallization conditions include quartz substrates, for example. However, quartz substrate is high cost, and processing a quartz substrate to have a large surface area is extremely difficult. Increasing the surface area of the substrate is indispensable particularly for raising mass production efficiency. Work toward increasing the surface area of substrates in order to increase mass production efficiency has been remarkable in recent years.
The processing of a quartz substrate into this type of large surface area substrate is difficult with present techniques, and even if it were possible, would not happen at present due to the costs of production beyond a profit. Glass is available, for example, as a material which can easily be manufactured into a large surface area substrate. A glass substrate referred to as Corning #7059 exists as this type of glass substrate, for example. Corning #7059 is extremely low cost, and is easily made into a large surface area substrate. However, Corning #7059 has a distortion temperature of 593° C., and heat treatment at 600° C. or higher causes a problem.
Corning #1737 is a glass substrate with a relatively high distortion temperature. The distortion temperature is high at 667° C. If an amorphous semiconductor film is formed on a Corning #1737 substrate, and the substrate is then placed in an atmosphere of 600° C. for 20 hours, there is almost no change in shape of the substrate which will influence manufacturing steps. However, a heat treatment time of 20 hours is too long to be used as a mass production process, and from the point of view of costs, it is preferable to lower the heat treatment temperature of 600° C., even by a small amount.
A novel method for crystallization has been proposed in order to resolve these types of problems. This method is recorded in detail in Japanese Patent Application Laid-open No. Hei 7-183540, and a simple explanation thereof is presented here. First, a very small amount of an element such as nickel, palladium, or lead is introduced into an amorphous substrate. Methods such as plasma processing, evaporation, ion injection, sputtering, and liquid application can be utilized as the introduction method. Thereafter, if the amorphous semiconductor film is placed, for example, in a nitrogen atmosphere at 550° C. for 4 hours, a crystalline semiconductor film having good characteristics can be obtained. The optimal heat treatment temperature and heat treatment time are dependent upon the amount of the element introduced and the state of the amorphous semiconductor film.
A method of crystalizing an amorphous semiconductor film in accordance with heat treatment is discussed above by way of an example. On the contrary, the temperature of the substrate does not increase very much with crystallization by laser annealing, and high energy can be imparted to only the amorphous semiconductor film. Therefore, it can be applied to substrates such as plastic substrates, in addition to low distortion temperature glass substrates.
Lasers such as XeCl excimer lasers and KrF excimer lasers can be given as examples of the types of lasers that can be used in laser annealing. A method of performing laser annealing in which: a pulse laser beam from a high output excimer laser is processed into a square spot of several centimeters in side, or into a linear shape having a length equal to or greater than 10 cm, on a surface to be irradiated by an optical system; and in which the laser beam is then scanned (or the laser beam irradiation position is moved relatively to the surface to be irradiated) has high mass-productivity and is industrially superior. This method is preferably used.
In particular, if a beam made from a laser beam having a linear shape (hereafter, referred to as a linear shape beam) in the irradiation surface is used, then the entire irradiation surface can be irradiated by scanning the linear shape beam only in a direction perpendicular to the longitudinal axis of the linear shape beam. This differs from the use of a spot shape laser beam, in which it is necessary to scan forward and backward, and to the left and right. Productivity is therefore high. Scanning in a direction perpendicular to the linear direction is performed because this is the most efficient scanning direction. In terms of its high productivity, pulse oscillation type excimer lasers, processed into a linear shape beam by a suitable optical system, for laser annealing are being mainly used in the present day.
Further, there is an additional method of performing crystallization of an amorphous semiconductor film by laser annealing, after crystallization is performed in accordance with heat treatment, in order to obtain a semiconductor film having very good electrical characteristics. The semiconductor film characteristics can be improved if this method is used, compared to cases of performing only heat treatment or only laser annealing.
Removal of a natural oxide film is often performed as a preprocess to laser annealing in order to prevent surface roughness. The composition and film thickness of the natural oxide film are uncontrolled, and may therefore be a cause of contamination and dispersion in characteristics. In particular, removal of the natural oxide film as a preprocess to laser annealing is also effective in removing portions having a high metallic element concentration for cases in which a metallic element is introduced into this type of amorphous semiconductor film, and laser annealing is performed after heat treatment. This is very important in order to obtain highly stable semiconductor films having little dispersion in their characteristics. A detailed explanation of this method is recorded in Japanese Patent Application Laid-open No. Hei 8-339960.
Further, it is known that convex portions, called ridges, can be formed in the film surface if laser annealing is performed on the semiconductor film, performing crystallization (such convex portions are hereafter referred to as “ridges”). The semiconductor film instantaneously melts and locally expands if laser light is irradiated to the semiconductor film, and the ridges are formed in the surface of the crystalline semiconductor film in order to relieve internal stresses that develop in accordance with this expansion. The elevation difference of the ridges is on the order of 0.5 to 2 times the film thickness.
Potential barriers and trap levels are formed in the ridges on the surface of the crystalline semiconductor film due to dangling bonds and lattice distortions, and therefore the boundary level between an active layer (semiconductor layer containing a channel forming region, a source region, and a drain region) and a gate insulating film becomes high for insulating gate type semiconductor devices. Further, the peak portions of the ridges are precipitously steep, and therefore electric fields are easily concentrated there and this becomes a generation source for a leak current. Finally insulation breakdown develops, and short circuits are formed. In addition, the ridges in the surface of the crystalline semiconductor film can damage the film coating properties of a gate insulating film formed by sputtering or CVD, and the reliability is lowered due to insulation defects and the like. Further, one factor in determining the electric field effect mobility for TFTs is the surface scattering effect. The levelness of the interface between the active layer of the TFT and the gate insulating film imparts influence to the electric field effect mobility, and the more level the interface is, the less influence by scattering is, and a high electric field effect mobility can be obtained.
From the above discussion, it can be said that it is preferable that the ridges be low. The ridges are made lower if laser annealing is performed in an inert gas atmosphere, compared to laser annealing performed in the ambient air atmosphere, but the crystal grain size becomes smaller, and the electrical characteristics of the TFTs become poor.
However, it has been reported that the ridges can be reduced while maintaining a large crystal grain size by performing laser annealing within the ambient atmosphere, performing hydrofluoric acid processing and removing an oxide film, and then performing laser annealing in an inert gas atmosphere.
Contamination due to metallic impurities and organic substances exerts a great influence on the electrical characteristics of semiconductor devices. Metallic impurity contamination causes defects such as oxidation film withstand voltage defects and a lowering in the carrier lifetime, and fatally degrades the electrical characteristics. For example, the surface of a silicon film is easily contaminated, and in particular, metals having a larger electronegativity than silicon directly take away electrons from silicon, chemically bond with silicon, and are difficult to remove. Metallic atoms that have a smaller electronegativity than silicon are not directly adsorbed onto the surface of a bare silicon film, but are oxidized more easily than silicon, and therefore are incorporated within a natural oxide film formed on the silicon film surface.
Organic substance contamination exerts influence on the electrical characteristics of oxide films. The withstand voltage can be reduced by forming an oxide film after removing organic substances on the surface of the semiconductor film by a method such as cleaning using a liquid mixture of sulfuric acid and hydrogen peroxide (hereafter, referred to as a sulfuric acid and hydrogen peroxide mixture) or cleaning using aqueous ozone. A solution in which sulfuric acid, H2SO4 (97%), and hydrogen peroxide are mixed at a composition ratio of 4:1 to 6:1 is often used as the mixture of sulfuric acid and hydrogen peroxide. Heat is generated at the same time when both liquids are mixed, reaching a temperature of 100 to 120° C. Further, the withstand voltage of the oxide film depends on the degree of organic substance contamination, and is a cause of dispersion in characteristics. A large amount of organic substances exist in the ambient air atmosphere of a clean room, and the amount of organic substances adsorbed increases with time. A large amount of moisture also exists, and if absorbed onto the film surface, the moisture promotes growth of a natural oxide film. It is also possible for metals and organic substances dissolved within the moisture to cause contamination.
It is necessary to remove a natural oxide film formed on the surface of a semiconductor film as a preprocess to laser annealing when performing crystallization by laser annealing after introducing a metallic element into an amorphous semiconductor film and then performing heat treatment. However, the surface of the semiconductor film is easily contaminated with impurities, which are difficult to remove. Semiconductor film contamination exerts great influence on the electrical characteristics of thin film transistors (TFTs) using the semiconductor film as an active layer, and can cause dispersion in the electrical characteristics.